Welded Fabrication of Nickel Alloys for Corrosion Resistant Service

요약:

This article focuses on the fabrication and welding of nickel alloys as they relate to the welders and production personnel engaged in fabrication of nickel alloys for corrosion service.
The physical properties of solid solution nickel alloys, nickel and nickel-copper solid solutions alloys, chromium-bearing solid solutions alloys, nickel-molybdenum alloys, are quite similar to the 300 Series austenitic stainless steels. The solid solution nickel alloys cannot be strengthened by heat treatment, only by cold working. The precipitation hardening nickel alloys are strengthened by special heat treatments similar to those for the precipitation-aging stainless steels.

This article focuses on the fabrication and welding of nickel alloys as they relate to the welders and production personnel engaged in fabrication of nickel alloys for corrosion service.

The physical properties of solid solution nickel alloys, nickel and nickel-copper solid solutions alloys, chromium-bearing solid solutions alloys, nickel-molybdenum alloys, are quite similar to the 300 Series austenitic stainless steels. The solid solution nickel alloys cannot be strengthened by heat treatment, only by cold working. The precipitation hardening nickel alloys are strengthened by special heat treatments similar to those for the precipitation-aging stainless steels.

Nickel and nickel-copper alloys

Welders discovered that the welding characteristics of nickel and, to a lesser degree, nickel-copper alloys are somewhat different from the chromium-bearing nickel alloys or the austenitic stainless steels. Primary among the differences is the low viscosity or inability of the molten weld metal to spread or flow in the joint; however, competent welders soon become accustomed to this and are able to produce quality welds. The materials engineer who is aware of this viscosity difference in advance is better prepared to cope with the false "materials problem" reports from uninitiated shop personnel.

Alloys 200 and 201. Nickel 200 and 201 differ in the amount of carbon, 0.15% maximum in Nickel 200 and 0.02% maximum in 201. Prolonged exposure of Nickel 200 in the temperature range of 425-650°C precipitates graphite. For this reason Nickel 201 is recommended for service in the 315-650°C temperature range. Nickel filler metal welds (ENi-1 and ERNi-1) are not subject to graphite precipitation and are used for welding both Nickel 200 and 201.

Equipment intended for caustic service provides an exception to the general rule that post-fabrication heat treatment is not normally required. A stress relief treatment of 700°C for 1/2 hour, followed by a cooling rate of 90°C (200°F) per hour is a standard procedure to relieve stresses as a safeguard against corrosion cracking in caustic service for these alloys.

Alloy 400 and R405. Alloy 400 is readily welded by all the common welding processes. Alloy R-405 is a free-machining grade of alloy 400, containing 0.025-0.060% sulphur and is available as rods or bars. Parts made of alloy R-405 usually involve little or no welding, but when welding is required, it is good practice to make generous filler metal additions and to minimize the amount of base metal melted, thus reducing the amount of sulphur in the weld. Alloy R-405 welds made with the SMAW process are often less affected by sulphur from the base metal than welds made by GTAW or GMAW.

Salt and brine environments. It is important to note that in salt or brine environments, alloy 400 matching composition welds may become anodic to the base metal and suffer galvanic corrosion attack. To solve this problem in brine environments, nickel-chromium type electrodes are used such as ENiCrFe-2 and ENiCrMo-3. Welds made with these electrodes are cathodic to the base metal and thus resist galvanic corrosion.

Hydrofluoric acid service. Welded alloy 400 equipment used in hydrofluoric acid service should receive a post-weld stress relief to avoid stress corrosion cracking. The stress relief treatment is performed at 540-650°C for one hour followed by slow cooling.

Chromium-bearing alloys

The nickel-chromium, nickel-iron-chromium, and nickel-chromium-molybdenum alloys may exhibit carbide precipitation in the weld heat-affected zone, a condition similar to that encountered in austenitic stainless steels. In most environments, however, the sensitization of these nickel alloys is not sufficient to affect the corrosion resistance; as a result, solution annealing is seldom required. Two factors function to reduce sensitization: very low carbon levels (as a result of recent improved melting practices), and the use of stabilizing additions of titanium and columbium in many alloys.

A post-weld heat treatment to prevent stress corrosion cracking is recommended when alloy 600 is used in high-temperature, high-strength-caustic-alkali service. The stress relief treatment is performed at a temperature of 900°C for one hour or at 790°C for four hours with a slow cool.

Nickel-molybdenum alloys

The materials engineer involved in fabrication of nickel-molybdenum alloy equipment should be aware of the background behind the three grades -- alloys B-2, B-3, and B-4, along with the precautions required in post-fabrication heat-treating. The welder will use matching filler metals for all three alloys and should detect no difference between the three alloys.

Alloy B-2 has been the standard nickel-molybdenum alloy for a number of years, having replaced the older alloy B. Alloy B had a shortcoming in that it required a solution anneal at a temperature of 1175°C after welding to eliminate carbide precipitates in the weld heat-affected zone and to restore corrosion resistance. A modification of the alloy composition resulted in the formulation of alloy B-2 which demonstrates acceptable corrosion resistance in the as-welded condition. This development made possible the construction of fabrications too large to be solution annealed.

Work with alloy B-2, however, revealed a problem: it experiences a phase transformation during brief exposure to temperatures in the range of 595-815°C. Such exposure can result in cracking during base metal manufacturing operations or annealing by fabricators after cold working. Recent alloy modifications by two different metal producers have overcome the 595-815°C low ductility problem and associated cracking of alloy B-2. The result of their work is the introduction of the two alloys: B-3 and B-4.

Precipitation-hardening nickel alloys

The precipitation-hardening nickel alloys are used in applications requiring corrosion resistance and a need for greater mechanical strength or higher hardness than is obtainable with the corresponding solid solution alloys. The precipitation-hardening or age hardening, as it is often called, is accomplished by the addition of increased amounts of titanium and aluminum along with special heat treatments. The heat treating temperatures vary from 600-760°C depending upon the alloy and specific properties desired. The hardenable alloys in the soft or solution-annealed condition have about the same strength as the comparable solid solution alloy.

General guidelines for nickel alloys

Preheat and interpass temperature. Preheat of nickel alloys is not required except to bring the metal in the area to be welded to room temperature or to a typical shop temperature to prevent moisture condensation. A maximum interpass temperature of 175°C is widely used although one base metal producer is more conservative and recommends a maximum of 95°C.

Post-weld heat treatment. In almost all instances, solid solution nickel alloys do not require a post-weld heat treatment for corrosion resisting service. Precipitation hardening alloys require heat treatment after welding to develop full strength. When heat treatment or stress relief is required for specific applications; for example, to anneal following cold forming, or for dimensional stability, the user should consult the nickel alloy producer’s literature or its staff for specific recommendations.

Prior to any heat treatment, it is essential that all alloy surfaces be thoroughly cleaned of oil, grease, paint, or markings, and similar contaminants to avoid catastrophic corrosion during heat treatment. The method of heating and cooling and the amount of sulphur in the furnace atmosphere must be controlled or the alloys can be damaged.

Filler metal selection for corrosive environments. Nickel alloys are normally welded with matching composition filler metals. In sea water and some environments, nickel-copper alloy welds made with matching composition filler metals may be anodic to the base metal and corrode preferentially by galvanic corrosion. This condition may be attributed largely to the fact that many "matching composition" filler metals are not of identical composition; some elements have been added or amounts adjusted for better weldability. Another factor to consider is that weld metal may also become anodic to the base metal as a result of segregation as it solidifies.

When experience demonstrates that matching composition welds corrode preferentially to the base metal, non-matching composition filler metals should be used that are both compatible metallurgical with the base metal and are cathodic to the base metal in the particular environment. Selection should be made by knowledgeable material specialist or by on-site evaluation tests. It is important to remember that most welding codes specify that the non-matching filler metal welds be treated as a dissimilar metal welds and indicate the need for a separate welding procedure specification and welding procedure test.

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